Non-traditional high temperature materials, such as ceramic matrix composite (CMC) materials, are more commonly being used for various components that are exposed to high temperatures. Exemplary CMC materials comprise ceramic fibers embedded within a matrix material such as silicon carbide, silicon nitride, etc., or combinations thereof. Because CMC materials can withstand relatively extreme temperatures, there is particular interest in replacing components within a flow path of combustion gases within a gas turbine engine with CMC materials. More particularly, a gas turbine engine generally includes a fan and a core arranged in flow communication with one another. The core of the gas turbine engine generally includes, in serial flow order, a compressor section, a combustion section, a turbine section, and an exhaust section. In operation, air is provided from the fan to an inlet of the compressor section where one or more axial compressors progressively compress the air until it reaches the combustion section. Fuel is mixed with the compressed air and burned within the combustion section to provide combustion gases. The combustion gases are routed from the combustion section to the turbine section. The flow of combustion gases through the turbine section drives the turbine section and is then routed through the exhaust section, e.g., to atmosphere. In general, turbine performance and efficiency may be improved by increased combustion gas temperatures. Therefore, there is increased interest in forming gas turbine components from CMC materials that can withstand such increased combustion gas temperatures.
However, forming components from CMC materials often presents several manufacturing challenges, including productivity, quality, and cost challenges. More specifically, typical CMC components comprise silicon carbide, requiring a process using diamond material to cut the CMC component. As an example, an ultrasonic machining process using a diamond flow may be used to cut, e.g., seal slots in a CMC component, but the diamonds mix with the CMC fibers and matrix material such that the diamonds are difficult to recycle. Accordingly, such processes are expensive. Further, component seal slots typically have high aspect ratios that make machining such slots difficult. For example, conventional cutting and grinding processes for seal slots are limited by tool deflection and speed as well as accessibility to tight corners and sides, and laser cutting processes are limited by the taper of the slot sides. Moreover, seal slots must have limited surface roughness to ensure high sealing efficiency and structural stability. However, laser machining processes can generate thermal stresses and micro-cracks in CMC components, often with a slot taper that is beyond the required tolerance, and conventional grinding and machining processes typically cannot machine slot corners, much less smooth corners.
As a result, electric discharge machining (EDM) processes generally are the most appropriate processes for defining features such as seal slots in CMC components. Nonetheless, EDM processing of CMC components does face its own challenges. For example, there is a differential removal rate between the ceramic fibers and the matrix material of the CMC component, with the matrix material removed at a higher rate than the fibers, which can generate irregular side surfaces, e.g., having fibers stick out of the surfaces of the slot sides. Further, a dielectric fluid usually is used, e.g., as a coolant in EDM processes. As the EDM electrode advances toward a desired slot depth, dielectric flushing may become increasingly difficult and may stall, and the electrode may be damaged by arcing. Additionally, because EDM is a thermal machining process, it also may generate micro-cracks, although the cracks generated in EDM processes generally are much shorter than cracks created in some laser processes.
Therefore, improved electrodes for EDM processes, as well as improved EDM processes, would be desirable. In particular, an EDM electrode that minimizes side discharges to help minimize surface roughness and micro-cracks would be beneficial. Moreover, an EDM electrode that optimizes electrode wear and side discharges would be advantageous. Additionally, a method for forming a slot in a CMC component that minimizes surface roughness and micro-cracks within the slot would be useful.